Router A Routing Table Contents with a Link Failure

Although a higher route cost is a more accurate reflection of the lower bandwidths offered by these alternative routes, it can introduce unwanted routing problems. In Figure 8-8, both of the T1 lines have failed and, therefore, cause both of the alternative routes to become active simultaneously.

Figure 8-8: Hop counts can add up to 16 too quickly.

Figure 8-8: Hop counts can add up to 16 too quickly.

Because both alternative links had a cost metric of 10, their simultaneous activation results in a route cost of greater than 16. The valid range for RIP's hop counter is from 0 through 16, with 16 representing an unreachable route. RIP ceases calculating routes above 15. Routes whose total cost would exceed 16 are declared invalid. Consequently, a notification (a triggered update) is sent to all immediately neighboring routers.

Obviously, this problem can be avoided by leaving the default cost equal to 1. If it is absolutely necessary to increment the cost metric of a given hop, the new cost value should be selected with great care. The sum total of the route between any given pair of source and destination addresses in a network should never exceed 15. Table 8-5 demonstrates the impacts of a second link failure on Router A's routing table.

Table 8-5:

Router Name

Host Name

Next Hop

Number of Hops

A

A.10

Local

0

A.15

Local

0

B.2

Gateway

11

B.9

Gateway

11

C.5

Gateway

16

C.20

Gateway

16

0 0

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